A hierarchical spatial assembly approach of silica-polymer composites leads to versatile silica/carbon nanoparticles

Assembly of silica and polymer in the absence of surfactant templates is an emerging strategy to construct intricate nanostructures, whereas the underlying mechanism and structural versatility remain largely unexplored. We report a hierarchical spatial assembly strategy of silica-polymer composites to produce silica and carbon nanoparticles with unprecedented structures. The assembly hierarchy involves a higher length scale asymmetric A-B-A core-shell–type spatial assembly in a composite sphere, and a nanoscale assembly in the middle layer B in which the silica/polymer ratio governs the assembled structures of silica nanodomains. Through an in-depth understanding of the hierarchical spatial assembly mechanism, a series of silica and carbon nanoparticles with intriguing and controllable architectures are obtained that cannot be easily achieved via conventional surfactant-templating approaches. This work opens an avenue toward the designed synthesis of nanoparticles with precisely regulated structures.


Fig. S1
. Synthesis process of AI-B-AO core-shell type of silica-polymer composite.Three steps (I) is included: In step 1, the polymerization of 3-aminophenol and formaldehyde in the presence of EDA leads to the formation of polymer inner core (AI) (65) with a Schiff base doped APF polymer network under alkaline condition (23) (II).In step 2, the hydrolysis and condensation of added TEOS precursors generate silica primary particles (48-50) (III), which triggers the formation of the middle layer (B) by silica-polymer self-assembly assisted by the EDA via electrostatic interaction (Ⅳ).In step 3, the polymer outer shell (AO) is formed by the remaining APF polymer.From TEM, the size of PC-0 is 115.2 nm.After adding EDA, the size of PC firstly increases to 170.0 and 176.9 nm (PC-1, PC-2), then decrease continuously from 172.8 nm (PC-3) into 84.6 nm (PC-7).The size trend of PC measured from DLS is similar to the one of TEM, as shown in grey line, the size of PC-0 is 154.9 nm.After adding EDA, the size firstly increases to 189.6 and 196.7 nm (PC-1, PC-2), then decrease continuously from 188.7 nm (PC-3) into 124.4nm (PC-7).Hollow inner cavity sizes of SNP-x measured from TEM images in Fig. 4 , A1-A8).The weight loss before 100 °C in the first region is attributed to physical adsorbed water.The second region from 100 to 700 °C is mainly attributed to the thermal decomposition of polymer and condensation of silica.The weight loss after 700 °C is negligible, due to stable silica structure.Near-spherical particles with a mean size of 2.41± 0.47 nm were observed when only TEOS was added into the reactive solution after 15 minutes, indicating the existence of SPP.The particle size was measured via Image J software based on the TEM image.The polymer can be fully decomposed after calcination in air, while silica is thermally stable.The absence of the nanoparticles with a larger size (~150 nm) after calcination indicates that they are polymers rather than silica.In bright-field TEM images, usually silica shows darker contrast than polymer.Randomly distributed silica islands with spherical morphology were observed on the surface of preformed APF core in SPC-1 at 2 min (Fig. S15A1), while the density of silica islands increased at 5 min (Fig. S15A2).Later, APF polymer was filled into the interspaces between these silica islands which was more obvious at 30 min (Fig. S15A4) where the rough silica/polymer nanosphere derived from exposed silica turned into smooth nanosphere.At the same reaction time, a higher density of silica islands with relatively smaller sizes were observed in SPC-4 (Fig. S15, B1, B2) than SPC-1.Besides, the APF polymer co-assembled with silica at early stage (Fig. S15, B1-B3) which could be observed from the relative smooth outer surface of silica-polymer composite nanosphere compared to SPC-1.The surface roughness observed in SPC-1 and SPC-4 was not obvious in SPC-7 (Fig. S15, C1-C3) indicating the highly polymer rich outer surface of SPC-7 and the much earlier co-assembling behavior of silica and polymer compared to SPC-1 and SPC-4.

Fig. S16. Time-dependent compositional analysis of SPC-1, 4, 7.
HAADF-STEM images (A1-C1 and A5-C5), elemental mapping images of silica (A2-C2 and A6-C6), polymer (A4-C4 and A7-C7) and the merged images of silica and polymer (A4-C4 and A8-C8) in SPC-1,4,7 at 5 and 15 min.The distribution of silica is represented by the silicon element with red color, the distribution of APF polymer is represented by the nitrogen element with green color which is derived from amine groups in APF polymer.
To differentiate the distribution of polymer and silica in the composite, HAADF-STEM and elemental mapping images were recorded for intermediate SPC-1,4,7 obtained at 5 and 15 min.For SPC-1 obtained at 5 min, the spherical silica islands with brighter contrast were observed in HAADF-STEM image (Fig. S16A1).The corresponding elemental mapping images (Si, N and Si+N, Fig. S16, A2, A3 and A4, respectively) shows that the polymer core is surrounded by spherical nanodomains, consistent with the findings in Fig. S15.The silica rich outer shell is also seen for SPC-1 obtained at 15 min (Fig. S16, A5-A8).For SPC-4 (Fig. S16, B1-B8), the size of silica nanodomain is smaller while the density is higher compared with SPC-1.Moreover, the radially aligned silica nanodomain is more obvious at 15 min (Fig. S16B5) than that at 5 min (Fig. S16B1).In addition, the overlap between silica and polymer is more even than SPC-1, indicative of simultaneous silica-polymer assembly.For SPC-7, the silica nanodomain size is the smallest among three SPC samples under study.The silica and polymer distribution are more homogeneous, and the aligned growth of silica is not evident (Fig. S16, B1-B8).The hollow inner cavity size of silica nanoparticles was measured to be ~80 and ~ 150 nm when the APF polymerization time was 15 and 50 min, respectively.The average particle size was ~250 and ~275 nm, respectively.By comparing the cavity and particle size of SNP-4 (Fig. 4A4) synthesized at the APF core formation time of 30 min (115 and 254 nm, respectively, Fig. S7 and Fig. 3A), it is concluded that the size of APF core increases with the polymerization time, leading to enlarged cavity size in the silica nanoparticles and reduced spiky length due to over consumption of APF in the inner core.When the amount of ammonia hydroxide added in the synthesis was 1.0 and 1.8 mL, silica nanoparticles with similar rodlike surface nanotopography were obtained.The particle size / cavity sizes were measured to be ~446 nm and ~ 196 nm at ammonia hydroxide amount of 1.0 mL, and ~ 211 nm and ~ 92 nm at ammonia hydroxide amount of 1.8 mL.It is noted that at the ammonia hydroxide amount of 1.56 mL, silica nanoparticles (SNP-4, Fig. 4A4) with an average particle size of ~ 254 nm and cavity size of ~ 115 nm were obtained (Fig. 3A, Fig. S7).The above observations indicate that the concentration of ammonia hydroxide used in the synthesis is another parameter that can adjust the structure of silica-polymer assembly.Increase in the overall solution alkalinity leads to faster nucleation of APF core with higher numbers and smaller sizes, thus both the particle and cavity sizes are reduced in the resultant silica nanoparticles.The DNA adsorption capability was evaluated via Nanodrop.As shown in Fig. S21A, SNP-5 showed comparable DNA adsorption ability with SNP-3, 4, 6 (also with 1D rodlike surface nanotopography), but significantly higher DNA adsorption ability compared with SNP-7, 8 (with disordered porous structure) and SNP-1, 2 (with spherical and ellipsoidal topography, respectively).Gel-retardation assay was further performed to study the binding affinity of pDNA by these SNP-x.As shown in Fig. S21C, SNP-3~6 showed a complete DNA binding at pDNA/SNP weight ratio of 1:40.The percentage of unbonded pDNA in all SNP-x at weight ratio of pDNA/SNP weight ratio of 1:10 and 1:20 was quantified based on the DNA band intensity through Image J software.As shown in Fig. S21B, SNP-5 exhibited the lowest DNA release percentage compared with other SNP-x at pDNA/SNP weight ratio of 1:10 and 1:20.At the same particle concentration of 60 g/mL, there is no significant difference in the silicon content per cell (Fig. S22A) of SNP-4, 5, 6, 7, 8. However their cellular uptake ability was significantly higher than SNP-1, 2, 3 samples, presumably due to the low specific surface area (< 100 m 2 /g , Table S1) of SNP-1, 2, 3 that are not beneficial for PEI modification and cellular uptake (6).As shown in Fig. S22B, the zeta potential value of SNP-1, 2, 3 (19.9,21.0 and 21.0 mV, respectively) was generally lower than that of other SNP samples (all above 28.5 mV).Although SNP-7 and SNP-8 possess the highest specific surface area (454.9 and 672.7 m 2 /g, respectively), their relatively lower zeta potential value compared to SNP-5 and SNP-6 may be attributed to the ultrasmall pore size (< 2 nm) that is not beneficial for PEI modification.

Fig. S7 .
Fig. S7.Analysis of particle and hollow cavity sizes.The particle sizes of PC-x (x=0~7, 0 is the sample without adding EDA) measured from TEM images (black curve) and via DLS (grey curve).From TEM, the size of PC-0 is 115.2 nm.After adding EDA, the size of PC firstly increases to 170.0 and 176.9 nm (PC-1, PC-2), then decrease continuously from 172.8 nm (PC-3) into 84.6 nm (PC-7).The size trend of PC measured from DLS is similar to the one of TEM, as shown in grey line, the size of PC-0 is 154.9 nm.After adding EDA, the size firstly increases to 189.6 and 196.7 nm (PC-1, PC-2), then decrease continuously from 188.7 nm (PC-3) into 124.4nm (PC-7).Hollow inner cavity sizes of SNP-x measured from TEM images in Fig.4are shown in blue curve.
Fig. S7.Analysis of particle and hollow cavity sizes.The particle sizes of PC-x (x=0~7, 0 is the sample without adding EDA) measured from TEM images (black curve) and via DLS (grey curve).From TEM, the size of PC-0 is 115.2 nm.After adding EDA, the size of PC firstly increases to 170.0 and 176.9 nm (PC-1, PC-2), then decrease continuously from 172.8 nm (PC-3) into 84.6 nm (PC-7).The size trend of PC measured from DLS is similar to the one of TEM, as shown in grey line, the size of PC-0 is 154.9 nm.After adding EDA, the size firstly increases to 189.6 and 196.7 nm (PC-1, PC-2), then decrease continuously from 188.7 nm (PC-3) into 124.4nm (PC-7).Hollow inner cavity sizes of SNP-x measured from TEM images in Fig.4are shown in blue curve.

Fig. S10 .
Fig. S10.TGA profiles of SPC-x (x=1~8, A1-A8).The weight loss before 100 °C in the first region is attributed to physical adsorbed water.The second region from 100 to 700 °C is mainly attributed to the thermal decomposition of polymer and condensation of silica.The weight loss after 700 °C is negligible, due to stable silica structure.

Fig. S11 .
Fig. S11.The impact of TEOS on the growth of SPC-8.(A) Photos of the reactive solutions of SPC-8 with (1#) and without (2#) adding TEOS in the second step.TEM images of SPC-8 with (B) and without (C) adding TEOS in the second step.

Fig. S13 .
Fig. S13.The role of EDA in the HSA mechanism.TEM images of separated silica and polymer spheres without using EDA (A) and silica nanoparticle after calcination to remove the polymer (B).The average size of polymer in (A) is ~150 nm while that of silica in both (A) and (B) is ~ 50 nm.

Fig. S17 .
Fig. S17.Control over the pore openings of carbon nanoparticles.(A) The XPS survey spectra and (B) corresponding Si/N ratio of the intermediate SPC-4 obtained at the reaction time of 5, 15 and 180 min.(C1, C2) SEM and (D1, D2) TEM images of carbon nanoparticles, and (E1, E2) TEM images of silica nanoparticles derived from intermediate SPC-4 obtained at 5 and 15 min, respectively.

Fig. S18 .
Fig. S18.The reproducibility of the synthesis method.TEM images of SNP-4 obtained in three different batches (A~C).The cavity and particle sizes (D) measured from ~50 particles in TEM images via Image J software.The bars are shown as mean ± SD, statistical analysis conducted by t-tests with p > 0.05 showing no significant difference (ns).

Fig. S19 .
Fig. S19.The impact of polymer core formation time on the structure of silica nanoparticles.TEM images of SNP-4 synthesized at the APF core formation time of 15 (A) and 50 min (B).

Fig. S20 .
Fig. S20.The impact of ammonia amount on the structure of silica nanoparticles.TEM images of SNP-4 obtained via changing the amount of ammonium hydroxide into 1.0 mL (A) and 1.8 mL (B).

Table S1 .
Textual properties of SNP-x (x=1~8, A1-A8) Note: S is BET specific surface area, D is average pore size estimated from the pore size distribution curves; V is total pore volume.Type or paste caption here.Create a page break and paste in the Table above the caption.

Table S2 .
Textual properties of CNP-x (x=1~8, A1-A8) Note: S is BET specific surface area, D is average pore size estimated from the pore size distribution curves; V is total pore volume.Type or paste caption here.Create a page break and paste in the Table above the caption.